In some applications, such as in the control of electric AC motors, more than one analog input voltage has to be measured and converted into a digital value by an analog-to-digital converter (ADC). In the case where only one ADC is available, different analog input values have to be connected to the input of the ADC one after the other. In AC motor control applications, this is a common scenario where the phase currents are measured via independent shunt resistors.
In other applications, it is also possible that one analog input value is used as input to the ADC, but the operating mode (e.g., different signal conditioning settings, such as input amplification) of that analog input value is changed at specific points in time. In AC motor control applications, this is also a common scenario where the phase currents are measured via a shared shunt resistor. Furthermore, the switching of the power switches in an AC motor may also introduce noise, and different switching patterns of the power switches define different currents to be monitored via the common shunt resistor. For example, with a first switching pattern (first operating mode), the phase current of a first phase can be measured via the common shunt, and with a second switching pattern (second operating mode), the phase current of the second phase can also be measured via the common shunt.
In such cases, the voltage at the analog input can change significantly between different operating modes (here, the definition of operating mode may also comprise the change of the input signal as well as change of signal conditioning parameters). Especially in applications where more than one voltage is measured at a common shunt resistor, the input voltages may change significantly.
In some applications the input voltage can be directly used as input for an ADC, whereas in other applications there is a need for additional amplification or fast level checks by comparator units. These functions may be handled by an input stage associated with the ADC.
FIG. 1 shows a typical voltage to be measured. The voltage is in the range of a few hundred mV and is converted to a digital value. At the switching time point (e.g., the change of operating mode of the input signal), overshoots can occur in the range of a few volts. Furthermore, the voltage to be measured, (i.e., common mode voltage) may also vary by a few volts between the different operating modes.
The reaction time to changes in the input signal of the input stage (i.e., the time between the change of an operating mode and the point where, e.g., an ADC can deliver a correct result or a comparator output is valid) should be minimized to allow fast reaction of higher level control loops or to avoid a “blind window” when checking the input voltage against thresholds, e.g., for overcurrent events. The reaction depends on the noise and unintended overshoots when changing the operating mode of the input signal. The reaction time of the system plays an important role in the system architecture and thus should be minimized.
Furthermore, the implementation of a measurement unit (i.e., the input of the ADC or signal conditioning unit such as the input amplifier shown below in FIG. 2) should be feasible in a standard technology without special high speed or high voltage devices. The measurement should also have an anti-aliasing filter with overload limitation (i.e., clamping properties).
FIG. 2 is a high-level block diagram illustrating a conventional circuit 10 for a standard shunt measurement with high current shunts. The conventional shunt circuit 10 enables the measurement of a chip external shunt voltage. The conventional shunt circuit 10 may include a shunt resistor 12 through which a shunt current runs, a preamplifier 14 coupled to the shunt resistor 12, and a passive RC filter, 16, 20. An Analog-to-Digital Converter (ADC) 18, that can be located on a microcontroller or other control device, is coupled to the conventional shunt circuit 10.
The shunt current, Is, may be in the range of a few Amperes to 100 Amperes and may generate a voltage Vmeas of a few hundred mV. This voltage may be fed to the preamplifier 14 which generates an output voltage with a full scale range of typically 3V to 5V. This voltage is then filtered by the passive first order anti-aliasing filter 16, 20 and converted by a conventional Analog-to-Digital Converter (ADC) 18 to a digital value. The amplifier of the conventional shunt circuit 10 also has a clamping property which limits the output voltage to a maximum of 3V to 5V (depending on the maximum input voltage of the ADC), thus avoiding a strong overload of the anti-aliasing filter at high overshoots appearing at the shunt. A strong overload causes a long recovery time which is not acceptable for certain applications.
Conventional shunt circuits such as that shown in FIG. 2 have several disadvantages. The conventional shunt circuit 10 of FIG. 2 requires a very fast and accurate preamplifier, external components and an input channel for the ADC. The preamplifier of the conventional shunt circuit 10 is difficult to implement into a standard control device (e.g., a microcontroller) and requires a lot of area and current, thus significantly increasing the costs associated with the control device. Another stringent requirement of the preamplifier is a huge common mode input voltage range varying in the range of a few volts (here, “huge” means that the required common mode range is much bigger than the input signal range of the measurement voltage due to current flowing through the shunt resistor). That means that the common mode input voltage can be a factor of 10 higher than the measurement voltage. Additionally, the implementation in FIG. 2 shows two different integrated circuits for the preamplifier and the ADC which consumes a significant amount of area and power. Finally, conventional ADCs for shunt measurements are often located directly on the control device (e.g., a microcontroller) whereas the shunt resistor is normally located close to the power switches. As a consequence, the analog input signal “sees” a long way from the shunt to the input of the control device with all known drawbacks, such as induced noise.
Therefore, there exists a need for a system and a method for shunt measurements which overcomes these disadvantages. More specifically, there is a need for a method and system for a passive input filter with adjustable clamping for shunt measurements that require less area and less power yet increases accuracy, speed and efficiency.